This program project is concerned with three general areas of neurobiological investigation each of which combines studies of structure and function in attempts to provide correlations useful in defining the elementary processes underlying the organization of complex neural activities. Studies will be carried out on neuronal development, interaction and organization. A wide variety of species and preparations will be employed in order to examine: 1) the basis for electrotonic coupling between neurons in systems where coupling is demonstrable and 2) the contribution of synaptic vs. ephaptic interactions in neuronal organizations subserving highly synchronized activities. Morphophysiological studies of developing neurons and synapses in tissue culture, and in vivo systems will be carried out with emphasis on the role of hormonal influences in altering developmental features. Electron microscope as well as histochemical studies of synapses will be carried out in conjunction with electrophysiological investigations of synaptic activities and developing neural systems in different vertebrate species. Studies of the mechanism of hormone actions on developing neurons and synaptic complexes will also be pursued. Neuroendocrine relations will be studied in other aspects of this Program and investigations will be continued on the organization of subcortical systems regulating sensorimotor activities in the mammalian brain. BIBLIOGRAPHIC REFERENCES: Fox, G.Q., Pappas, G.D. and Purpura, D.P. Fine structure of growth cones in medullary raphe nuclei in the postnatal cat. Brain Res., 1976, 101: 411-425. Fox, G.Q., Pappas, G.D. and Purpura, D.P. Morphology and fine structure of the feline neonatal medullary raphe nuclei. Brain Res., 1976, 101: 385-410.